1. Field of the Invention
This invention relates generally to the formation of a shape in a metal workpiece. More particularly, this invention relates to the formation of a shape such as a cavity in a metal workpiece. The cavity may be defined by a base surface which is generally parallel to the base of the workpiece and which may have a bore and/or counterbore extending into the base which is of a predetermined configuration.
1. BACKGROUND INFORMATION
The present invention has particular utility in forming a metal workpiece which may be used as a component for metal electronic packages. Such packages are shown and disclosed in U.S. Pat. No. 4,897,508. However, it is to be understand that the present invention has broader application and may be used for forming other types of workpiece by the displacement of metal to form a shape therein.
In one form of a metal electronic package, the package comprises two parts, a base component and a cover or lid component. The base component is generally flat without sidewalls and a leadframe is spaced from the base member by a sealant such as an epoxy so it is in a plane which in alignment with the upper surface or bonding face of the electronic device which is attached to the base member. The wire bonds connecting the bonding face of the metal package and the lead frame may flex upwardly so the upper component or lid of the package includes a cavity in its interior surface to provide the space for the flexing while minimizing overall thickness of the package.
One method of obtaining a cavity in a metal workpiece is by stamping. However, when there is a requirement that the impression exceed 15 to 20 percent of the metal thickness, the metal may exceed the shear limit and tend to crack or fracture at the inner edges of the cavity. Additionally, forming the cavity by metal deformation as by stamping requires a large volume of metal to be displaced with the resulting difficulty of achieving the required degree of parallelism.
Other methods of forming cavities in metal include such processes as milling or chemical etching. The disadvantages with metal removal processes of this type are the time required to form each cavity and the metal scrap and waste products. The metal removed rapidly becomes a problem due to the cost and difficulty of disposing the small metallic chips formed as a by-product of the milling operation. In chemical etching, the storing and disposal of the chemicals and waste products produced presents problems.
Still another method of forming a cavity in metal is by imprint rolling. However, this type of process does not provide proper containment of metal flow.
U.S. Pat. No. 4,928,512 discloses another method of forming cavities in a metal workpiece in which a series of punches are used to form a cavity in the workpiece. The series of punches are made up of alternating piercing punches and flat impact punches.